1. Field of the Invention
This invention is concerned with the ion-exchange treatment of zeolitic particles, in particular to the ion-exchange treatment of zeolitic particles of a size and shape useful in fluidized bed processes, such as for example in fluid cracking catalyst (FCC) processes. In particular, the invention is concerned with an improved method and system for ion-exchanging cations such as sodium ions in zeolitic microspheres on a continuous commercial basis which features the use of moving sectionalized vacuum belt filters operated to include flooded percolation zones and in which ion-exchange treatment takes place with a countercurrently flowing ion-exchange liquid such as a solution of an ammonium salt.
2. Prior Art
Staged countercurrent ion-exchange is old in the art. When employed in conjunction with a series of thickeners effective exchange can be carried out. However, when applying conventional staged countercurrent ion-exchange to replace sodium ions with ammonium ions in zeolitic catalyst precursor microspheres (such as those produced by the process of U.S. 3,647,718 to W. L. Haden, Jr. and Frank J. Dzierzanowski), very dilute solutions of exchange salts are required. To reduce the sodium content of the zeolitic microspheres to a desired level, for example 1 percent or below, large volumes of exchange solution are needed. Adaptation of the conventional procedure to commercial practice would create large volumes of dilute spent nitrate solution, resulting in a costly concentration and/or disposal problem. Further, it would necessitate a large investment in equipment and plant site because of the large size of the equipment needed.
These obstacles were overcome by carrying out the ion-exchange treatment by the unique ion-exchange technique described in U.S. Pat. No. 3,943,233 of David B. Swanson and Walter L. Haden, Jr. This patent discloses a process for carrying out a plurality of liquid-solid contact steps on a single continuous vacuum belt filter wherein the rate constant of one of the steps, such as ion-exchange reaction, is an important factor. In an embodiment of the invention, exchangeable sodium cations in fluidizable zeolitic cracking catalyst precursor microspheres are exchanged with ammonium ions on a single sectionalized vacuum belt filter with a subdivided vacuum box underneath the upper surface of the belt. The microspheres are slurried, preferably in a spent ion-exchange solution, and the slurry is continuously fed at a substantially constant rate to the feed end of a continuous horizontally moving belt vacuum filter. While on the belt the microspheres pass serially through a cake-forming zone, then into an elongated ion-exchange zone and finally to a washing zone. The washed cake is discharged from the belt as it passes over a pulley system and the belt is sprayed with water on its return trip under the unit in order to remove adherent microspheres which would cause the belt to wear. A feature of the process of U.S. Pat. No. 3,943,233 is that the microspheres are formed in the first zone into a cake having a substantially dry crack-free surface but containing liquid in the voids between the microspheres. This condition is established by controlling the depth of vacuum in the first zone. The ion-exchange zone is operated under conditions such that the exchange solution percolates through such cake at a controlled rate of flow. The cake which leaves the percolation zone has a smooth substantially crack-free dry surface and contains liquid in the voids between the zeolitic microspheres. This condition is established by applying a relatively low vacuum depth under this zone of the filter. By applying a high vacuum under the third zone, wash liquid rapidly passes through the ion-exchange cake. In an illustrative example, the procedure was used to reduce the sodium in zeolitic catalyst microspheres (obtained by the process described in U.S. Pat. No. 3,647,718, supra) from an initial level of about 10 percent to about 1 percent by ion-exchange with hot 2N ammonium nitrate solution.